This invention relates to powder spray systems, and, more particularly, to a system for detecting fire within a powder spray booth and initiating a shut down of the operation of the booth to extinguish the fire.
The process of coating products with a solid resin coating involves preparing the resin coating in a finely ground powdered form and spraying it onto the products in a manner similar to liquid paint. Conventionally, an electrostatic charge is applied to the sprayed powder to enhance the attraction of the powder to the products which are held at ground potential. The electrostatic charge maintains the powder upon the part for a sufficient time period to permit the powder to be heated so that it melts, and when subsequently cooled, is firmly attached to the product.
In most applications, powder deposition is performed in a booth which provides a controlled area from which any oversprayed powder not deposited on the article is collected. Spray guns mounted in the booth and connected to a source of air-entrained powder, such as a powder feed hopper and powder pump, spray powder onto articles moving through the booth which are suspended from an overhead conveyor. Hooks or other supports carried by the overhead conveyor support the articles within the booth and maintain the articles at ground potential to attract the electrostatically-charged powder discharged from the spray guns.
Solid resin coating in finely ground, powdered form is highly combustible and can be ignited within the spray booth by sparks from an article which is not properly grounded. The powder supplied from the spray guns, the powder deposited onto the articles and the powder collected on the sides and floor of the booth all provide a source of fuel for a fire in the spray booth. If such fires are not extinguished immediately, the booth, spray guns and auxiliary equipment can be severely damaged.
Fire detection systems for powder spray booths have been in use for several years to detect a fire within the booth and immediately shut down all systems associated with the booth, including the supply of powder to the spray guns, to extinguish the fire. One of the most commonly used fire detection systems is an ultraviolet light or UV detector mounted at the ceiling of the booth and operatively connected to a controller which controls the powder spray booth systems, including the exhaust system, the high voltage supply, the compressed air supply, the product conveyor and the powder feed system.
Fire and electrostatic sparks produce a narrow band or ultraviolet light. The UV light detectors are tuned to this band so that when a fire is ignited within the booth, the UV detector senses the fire and immediately sends a signal to the controller which shuts down all of the systems associated with the powder booth. When properly used, UV light detector systems have proven effective in quickly extinguishing fires within powder spray booths.
One limitation of UV fire detection systems is that the cost is often prohibitive for smaller spray booths employing two or three spray guns. In such applications, the UV light detection system is often more expensive than the powder application system. As a result, many operators of small powder booths have refused to employ any fire detection system and this has resulted in damaging fires.
Another problem with UV light detection systems relates to their sensitivity. As mentioned above, UV light detectors are tuned to detect the small band of ultraviolet light produced by a fire or an electrostatic spark and then send a signal to the controller to shut down all systems associated with the booth. In some instances, an improperly grounded article will produce sparks which can be detected by the UV light detector but which are not situated within a cloud of dust concentrated enough to start a fire. The UV detector sends a signal to the controller to shut down the booth systems, but no fire has occurred in the booth.
In another case stray UV light from an electric arc welding operation or from sparking electrical contacts in nearby industrial equipment may be detected and cause a shut down.
In these instances, the operator of the booth may assume that the UV detector is not functioning properly because no fire had started in the booth. To continue with production, the operator often bypasses the UV detector and then runs the booth without any fire detection system. If a fire now occurs, there is no provision for automatic shut down of the booth operation and the fire can cause substantial damage to the booth and spray guns before it can be manually extinguished.
One attempt which has been made to simplify and reduce the cost of fire detection systems involves the placement of a fusible or burnable line near the ceiling of the booth which is connected to a switch adapted to send a signal to the controller of the spray booth. In the event of a fire in the booth, the line will burn through and activate the switch so that a signal is sent to the controller to shut down the booth operation.
The problem with this approach is that most of the fires in powder spray booths occur at or near the powder spray guns because they are closest to an improperly grounded article which can produce sparks and are discharging large quantities of powder onto the article. A substantial amount of damage to the booth and spray guns can occur in a fire beginning in the area of the spray guns before it finally reaches the ceiling of the booth and burns the detection line of such prior art detection system.